This is a regression caused by T240487.
The behaviour was correct before but it needs to do that while avoiding the XSS that it tried to fix.
I dont think we should support HTML in overlay headers. Having a link or any styling in their is unnecessarily confusing.

I'd recommend using

			section: new mobile.Section( {
				line: $('<span>').text( $($headline).text() )[0].outerHTML,
				text: $heading.next().html()
			} ),

Ah, okay. However I notice such links are visible but not clickable in the section title when it's collapsed, the action to open the overlay takes precedence. That makes sense, but if the link is removed from the overlay header then it will leave no way to open such a link at all.

Yeh section headings are pretty problematic on mobile due to the fact that clicking on one is what toggles open sections. I'd love to understand why they are used in place of a link/hatnote at the top of the section.

Change 572730 had a related patch set uploaded (by Jdlrobson; owner: Jdlrobson):
[mediawiki/skins/MinervaNeue@master] Do not allow raw HTML in talk page overlay header

https://gerrit.wikimedia.org/r/572730

Fix pushed.

That makes sense, but if the link is removed from the overlay header then it will leave no way to open such a link at all.

The "read as wiki page" option still allows access to the link.

Change 572730 merged by jenkins-bot:
[mediawiki/skins/MinervaNeue@master] Do not allow raw HTML in talk page overlay header

https://gerrit.wikimedia.org/r/572730